1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and in particular to a semiconductor device including a flat active region and a manufacturing method thereof.
2. Description of the Related Art
According to miniaturization of a semiconductor device, a semiconductor substrate surface shape with a miniaturized active region adversely affects characteristics of the semiconductor device.
The active region is separated by providing device isolation, for example, shallow trench isolation (STI), to a semiconductor substrate surface. After the device isolation is formed, a mask insulator used for forming the device isolation is removed. Due to formation of the device isolation and removal of the mask insulator, a non-flat portion is formed in the active region near a boundary between the device isolation and the active region. In a conventional semiconductor device, for example, a non-volatile semiconductor storage device having a floating gate electrode, a gate insulator (or a tunnel insulator) is formed on the active region having the non-flat surface, and the floating gate electrode is provided so as to cover the active region and touch the device isolation (i.e., see Jpn. Pat. Appln. KOKAI Publication No. 2004-48064).
The semiconductor device thus manufactured has a cross-sectional shape such as shown in W. H. Kwon; “Highly Reliable 256 Mb NOR Flash MLC with Self-Aligned Process and Controlled Edge Profile”, Proc. SSDM 2005, Kobe, pp. 448-449. An example of a cross-sectional shape of the conventional non-volatile semiconductor device having an active region separated by STI is shown in FIG. 1. A semiconductor substrate 10 surface of an active region AA is not flat wholly, and it has sagging near both ends thereof such as a portion shown by M. Further, a side face portion of the STI 20 projecting above the semiconductor substrate 10 retreats due to etching and a width thereof becomes narrower than a portion in the semiconductor substrate 10, so that a portion thereof contacting with the semiconductor substrate 10 sinks as shown by N in FIG. 1. A floating gate electrode 24 is formed over the active region on a gate insulator 22. That is, the floating gate electrode 24 is formed so as to cover end portions of the non-flat active region. Therefore, the floating gate electrode 24 has a cross-sectional shape projecting downwardly that it fills in the sink of the STI 20 near the end portion of the active region AA. If a MOS transistor is formed in a region having such a shape, since a channel region is not flat, for example, such a problem that electric field concentrates to the end portion of the active region during programming data to the floating gate electrode 24 where a high electric field is applied and a programming characteristic dispersion occurs.